The invention relates to microwave power divider circuity, and more particularly to monopulse power divider networks suitable for use in radar antenna arrays.
For an antenna array, the RF energy needed to excite the individual radiating elements originates from a single transmitter. The energy is then distributed to all the elements through the antenna feed network. To have the antenna operate across a wide instantaneous bandwidth, the feed network often uses a corporate architecture with matched four port power dividers (one port is terminated in a matched load) performing the RF power distribution.
To perform monopulse operation, the feeding of the array is typically split in halves (or quadrants), and then the halves (or quadrants) are combined together with a network of Magic-Tee couplers, as shown in FIG. 1. This network will provide an amplitude distribution across the array. However, the Magic-Tee couplers will create a 180 degree phase change between the two halves of the array at the difference port while maintaining equal phase for the sum port excitation. The results are two entirely different radiation patterns generated by the array. These two patterns are then used by the radar for monopulse tracking. The problem with this approach is that the optimal amplitude distribution required to generate a sum pattern with low sidelobes is very different from that required for low difference pattern sidelobes. Conventional monopulse and corporate feed networks can yield only one amplitude distribution.
One approach to realize separate sum and difference distributions is to have corresponding elements across the halves of the array connected together with Magic-Tee couplers fed by a separate sum feed network and difference feed network. Unfortunately this approach requires a formidable amount of excess transmission lines to connect all the components and thus resulting in depth and weight penalties.
Conventional monopulse networks are limited to two and four output ports. Their sum and difference channels excite only RF identical amplitude distributions with the only difference being phase reversal between ports. See, M. Skolnik, Introduction to Radar Systems, 1980, at pages 160-167. Ladder feeds have wide tunable frequency bandwidth and narrow instantaneous bandwidth, and require a number of different precision couplers. These couplers would either be machined slots in waveguide (expensive, bulky and bandwidth limited) or proximity couplers in stripline (requiring tight dimensional tolerances in the manufacturing process to make producible). Ladder feeds require a complicated conglomeration of additional couplers and Magic-Tee couplers to realize monopulse functions. The antenna beam will scan as a function of frequency with this feed. See A. Lopez, "Monopulse Networks for Series Feeding an Array Antenna," IEEE Tran. on Antenna and Propagation, Vol. AP-16, No. 4, July 1968, p. 436; J. Smolko, "Compact Highly Integrated Dual Linear Antenna Feed," Proc. of the 1990 Antenna Applications Sym., September 1990, p. 538.
Partitioned feeds using a waveguide conglomeration of magic-tees as used have narrow frequency bands and use excessive amounts of waveguide to interconnect all the individual components. "Practical Phased Array Antenna Systems," Eli Brookner, Artech House, 1991, at pages 6-6 to 6-9.